Currently, we have markdown files in many places (e.g. comments, new
issues, etc.). This Merge Request detects if the file being edited is a
markdown file and adds markdown buttons and their functionality to the
single file editor (Not the web IDE)

- Reduce font size
- Reduce padding and margin
- Remove unused SCSS variable

**Why?**
Commit 75496059 accidentally changed line height for all small buttons.
This fixes that by introducing some `btn-xs` variables.

Makes the following changes in order to have all the
items of the top nav fit within all of the new Bootstrap 4 breakpoints:

- Change GitLab logo to show only on desktop breakpoint (lg and up)
  - Move margin from logo to logo text, saving 8px
- Move the following icon links to the "More" nav dropdown
  - Instance Statistics (ConDev Index)
  - Admin area (if Admin)
  - Sherlock Transactions (if enabled)
- Removed separator between Admin icons
- Change search input width from 240px to 200px
- Move "Activity", "Milestone", and "Snippets" into "More" menu on lg breakpoints
- Show "More" menu of lg breakpoints

Continues the iteration on the project overview UI:

- moved star, fork and new clone button
(copy SSH/HTTPS URLs) to top right,
  made them smaller
- avatar is now larger (64px)
- 'Request access' is now a link instead
  of a button
- overview comes before the description +
  changed styling and added icons
- description font-size is now 16px (large-paragraph)
- quick links to files are moved downwards
  below the commit/pipeline info
- margins changed to group content into
  4 groups to clean up the interface
- visibility info reduced to icon-only and
  moved to the right of the title

- https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8593
- Create mr-widget-icon to encapsulate circle icons
- Decouple workflow L&F from mr-widget-heading
- Decouple container L&F from mr-widget-heading

Fixes the styling of image discussions after the discussion styling
changes.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/54110

Ensure that line numbers and the text content have the same line-height set.